Ecotoxicity evaluation of bromacil and tebuthiuron on native vegetation species in Alberta, Canada

Soil sterilants such as bromacil and tebuthiuron were widely used across Alberta from the 1960s to the late 1990s. These non-selective, residual herbicides inhibit vegetation growth and pose risks for off-site migration. Current Alberta Tier one soil guidelines are primarily based on agronomic species, which may not reflect the sensitivity of native vegetation. This study evaluated the toxicity of bromacil and tebuthiuron to five native grass species, with four species tested for each sterilant, using a modified Environment Canada test protocol. Toxicity endpoints included emergence, root/shoot biomass, and root/shoot length, with inhibition concentration values derived for each species and endpoint. Species sensitivity distributions for the soil-applied herbicides bromacil and tebuthiuron were developed following the Canadian Council of Ministers of the Environment protocol to estimate effect concentrations for protection of the ecological direct contact pathway. Native species evaluated in the current studies exhibited greater sensitivity to bromacil and tebuthiuron than those assessed in earlier guideline development efforts. This study provides critical data to support the sustainable management of sterilant-impacted sites in Alberta, informing potential guideline development.